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Some HETiosyncrasies
of HET Data Acquisition
What targets can HET access?
The HET can observe 81% of the sky that reaches elevation above
airmass=2.5 from its vantage of 30.68 degrees north latitude. That
declination range is about -11 to +72. The target airmass at
observation time can range between 1.14 and 1.33, with the typical
value being 1.22.
When can the HET service observer access them?
Unlike conventional telescope observing, which is naturally
organized by the progression of target right ascensions, HET
observing is naturally organized as sequencing of target "tracks."
A track consists of all the details in connection with the
celestial transit (or at extreme declination: culmination) across
the HET "donut" (almucantar) of pointing accessibility on the sky.
This is not so restrictive, for two reasons. A database program
organizes the track information (particularly track start, middle,
end) for convenient oversight. Also, there is in the north and
south azimuth quadrants, where celestial motions are more
horizontal than vertical, considerable leeway for setting the
structure azimuth displaced from the transit azimuth in order to
obtain nearly (>90%) the cumulative pupil available in the
transit geometry. This extends accessibility to earlier and later
sidereal times.
When will your program get access over competing programs?
After sheer celestial accessibility, the dominant factor is TAC
priority (0 strongest to 4 weakest), which most influences the
execution order among the many science targets of a given
trimester. The consequence is that strong priority (0 or 1)
targets generally receive visits early in the trimester or as soon
as accessible, middle priority (2 or 3) targets being more
concentrated toward mid-trimester, and low priority targets
significantly concentrated toward the trimester end.
This historic pattern has changed somewhat in the era of HPF, as
PIs seeking phase coverage utilize their lower priority time early
in the trimester and use their higher priority time near the end,
to fill in missing phases.
The HET design imposes certain constraints on the timing of
accessing parts of the celestial sky, compared with a conventional
telescope of similar aperture. Full productivity is however
redeemed by means of compensating design features: the queue
observing method and a large degree of flexibility in optimizing
efficiency via the on-the-fly choice of the most efficiently
observed targets, by folding in a number of time-variable factors
in pursuit of optimizing the use of the telescope at all times.
Thus unlike a conventional telescope of similar aperture, which
can opt to observe in a purely monotonic priority sequence,
sidestepping hard choices by resorting to higher and higher
airmasses ("target choice 1 AND target choice 2"), the HET
requires a more continual and active discrimination between
currently available targets ("target choice 1 OR target choice
2"). This is no real handicap so long as the time-dependent choice
modifying factors are given due consideration. The trade-off is
that priority, while dominant, is not the sole determinant factor
at any instance, but not to the extent that it becomes
meaningless.
With minimal elaboration, the time-dependent target choice
(non-priority) factors include the following:
Disfavorable discrimination factors:
- Very high wind, permitting visits to certain azimuth targets
instead of others.
- Degree of sky transparency, relative time smoothness of the
sky transparency, sky brightness, and seeing, permitting
certain science and not other.
- Unsteady conditions, in general tending to permit shorter
duration targets, instead of long duration ones that risk
wasteful failures.
- Detector equipment status, permitting certain science and
not other.
- Leads and lags in the time sequence of track accessibility,
permitting seamless (no wait) starting on some targets, and
thus preempting the starting of others.
- Leads and lags relative to the time of optimal pupil, such
that the visit to some targets would now be highly efficient,
whereas other targets will enjoy comparable efficiency only on
a future track or night (and there is ample expectation of the
latter).
- An already excessive number of instrument configurations
with time consuming calibrations (that have to extend well
into the daytime hours in conflict with other uses of the
facility), disfavoring engaging an additional instrument
configuration that particular night.
Favorable discrimination factors:
- Scientifically based time criticality of the observation,
e.g. synoptic targets that are becoming overdue relative to
the visit cadence specified, or pulsation or orbital phasing
requiring that particular track time.
- Diminishing availability (calendrical, lunar cyclic) time
criticality of the observation.
- Taking advantage of time dependent instrument
configurations, e.g. a grism due to be switched out or an
instrument setup that is currently set in position.
- Targets with many visits requested, as they near completion
can be favored to avoid the risk of a major incompletion case
at trimester end.
- If the queue is very congested, targets that permit
splitting of sub-exposures between different tracks can
sometimes permit execution.
- All else being equal, sequencing targets lying within a
small (3deg)
azimuth range, diminishes setting overheads.
- Targets requiring the absolutely rarest, best sky
conditions, may be purposely matched to those chance
occurrences.
What are the HET comparative advantages?
- The telescope geometry enforces a scheduling discipline
which nets an advantageously low typical airmass at time of
observation.
- It is easy to fine tune observing to a desired cadence or
pulsation or orbital phasing.
- Poor or borderline conditions do not cause any resource
wastage, unlike the expenses incurred for comparable visitor
mode observing.
- Observing efficiency and reliability benefit from continual
practice and upgradings.
How different are typical HET observing procedures?
HET observing broadly resembles conventional telescope observing
(with an observer/operator pair) except that the target choice
optimization has to be planned more proactively. The constraints
on HET productivity are slightly different for two reasons.
Firstly, the increased pupil cross-section in the middle reaches
of a track compared with the extremes, drives a more detailed
analysis of the scheduling. Secondly, there being no "Plan B" of
picking up targets from just anywhere on the sky, there is more
inherent risk of incurring queue gaps (in time and azimuth).
Although there is no official "filler target" mechanism, as the
queue wanes arrangements must sometimes be considered to prevent
any time or wind-azimuth dependent "hole" being forced upon the
telescope usage. Finally, being pure queue service mode, there can
be significant PI daily (and with TOOs, nightly) input to the
process via communication channels.
Last updated: Fri, 29 Dec 2023 12:26:35 +0000 stevenj
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